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Hollow-Core Fiber Gyroscope Operates on Cost-Effective Platform

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A consortium led by researchers at the Fraunhofer Institute for Reliability and Microintegration (Fraunhofer IZM) is using hollow-core optical fibers to develop a reliable method to transmit light to make gyroscopes less susceptible to interference caused by certain material properties, and/or by electrical or magnetic fields. Such improvements to the instrument are poised to support gyroscope performance in the aerospace sector, among other industries in which the sensor is deployed.

The team’s platform uses miniature collimators — highly precise lenses that capture light from one fiber and emit it before any diffraction can occur — to enable functionality.

Both hollow- and solid-core optical fiber are widely used in telecommunications. Optical fiber is also used in existing measurement technology — including in the precise rotation sensors known as gyroscopes.

In measurement technology, if only one axis of movement is relevant, acceleration sensors, known as accelerometers, are sufficient sensors. However, to track the movement of an autonomous object through all three dimensions of space, the measuring system must include three accelerometers and gyroscopes.

In the aerospace sector, for example, gyroscopes obtain measurement of light to check and stabilize the course of a vessel in flight.

Solid-core optical fibers contain a glass core through which light moves. As the refractive index of the material shrinks and the closer one gets to the outer layer, light bounces back from the fiber walls, moving through the inner core.

Depending on the direction of travel, time is lost or gained. A fiber gyroscope includes a fiber that is wound around a coil and forms a ring resonator. Light in the resonator travels in relation to time. When the object turns, the path passed by the lightwave changes imperceptibly, either shrinking or expanding by a tiny margin. A detector picks up this minute change to calculate rotation.

However, the researchers said, magnetic and electrical fields can interfere with the sensor’s interpretation in this process. Additionally, the material itself can interact with the light and cause a change in its optical properties.

These nonlinear effects directly influence how the light travels. Though the interference is considered minimal for telecommunications and poses no problem for that field, it can prove critical for navigating autonomous objects. A tiny deviation from the expected direction leads to a measurable deviation from the chosen course.
A team led by Fraunhofer IZM researchers showed that the use of hollow-core fibers makes fiber optic gyroscopes less susceptible to external interference factors. The demonstration supports gyroscope use in the aerospace sector, as well as other industries and applications in which optical elements that require free beam coupling are used. Courtesy of Fraunhofer IZM.
A team led by Fraunhofer IZM researchers showed that the use of hollow-core fibers makes fiber optic gyroscopes less susceptible to external interference factors. The demonstration supports gyroscope use in the aerospace sector, as well as other industries and applications in which optical elements that require free beam coupling are used. Courtesy of Fraunhofer IZM.
The researcher’s platform uses hollow-core fibers, which are as thin as typical, or solid-core optical fibers and contain air instead of a glass core. Light passes through that hollow space without disruption, which reduces the potential of material effects. Light also moves through the material at 1.5 times the speed of standard fibers, the researchers said.

In the work, Fraunhofer IZM researchers Wojciech Lewoczko-Adamczyk and Stefan Lenzky aimed to the take advantage of the disruption-resilient properties of hollow-core fibers to construct highly precise gyroscopes while also keeping costs down. The first needed an interconnection technology that could work with the fiber type, keeping in mind the challenge of splitting the light signal for multiple channels.

Typically, individual waveguides are coupled by fusing. However, the structure of hollow-core fibers is lost when exposed to heat.

The researchers constructed miniature collimators, which enabled the light to be split by half-reflective mirrors and fed into the ring resonator. After one trip around the ring, it is then measured and fed back into the fiber through a second collimator.

Tests showed that the optical components produced by Fraunhofer IZM outperform current solutions in the market with tenfold precision, at a maximum angle of refraction of 0.04°, with collimators still being used to bend the beams in the platform. This means that pairs of collimators can be used for the passive coupling platform without requiring additional alignment, while achieving coupling efficiency of more than 85%.

Although the components can be placed and aligned with precise positioning tools in laboratory environments, such tools are unlikely to be available in industrial production sites — which could explain why small to medium enterprises have been unable to offer this process to date, the team said. The researchers have moved to develop a passive coupling platform that allows the technology to be integrated in individual applications.

Its layout, they said, allows the precise fitting of the finished collimators, which reduces the need for additional alignment.

The project next aims to test reliability, add additional optical and mechanical components, and fit everything inside a gyroscope. Once the rotation sensor has been constructed, everything is ready to field test the technology under real-life conditions.

In addition to its potential to make aircraft and satellites more resilient to disruption, the collimator assembly platform could serve as a hybrid addition to integrated optical systems that use optical elements that require free beam coupling. Scattered light exiting a waveguide can be collimated to reduce losses when reentering the next waveguide.

The optical solution will also be relevant for materials processing with ultrahigh-powered light beams, or for transmitting infrared or shortwave UV light, according to the researchers.

Photonics Handbook
Sensors & Detectorssensorsgyroscopesfiber gyroscopesfiber optic gyroscopescollimatorsopticsoptical elementsprecision navigationaccelerometersbeam collimatorsdevicesmaterials processingFraunhoferFraunhofer IZMaerospace & defensepositioningpositioning and alignmentpositioning and sensingResearch & TechnologyeducationEurope

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